Problem: Is ${272673}$ divisible by $9$ ?
Answer: A number is divisible by $9$ if the sum of its digits is divisible by $9$ . [ Why? First, we can break the number up by place value: $ \begin{eqnarray} {272673}= &&{2}\cdot100000+ \\&&{7}\cdot10000+ \\&&{2}\cdot1000+ \\&&{6}\cdot100+ \\&&{7}\cdot10+ \\&&{3}\cdot1 \end{eqnarray} $ Next, we can rewrite each of the place values as $1$ plus a bunch of $9$ s: $ \begin{eqnarray} {272673}= &&{2}(99999+1)+ \\&&{7}(9999+1)+ \\&&{2}(999+1)+ \\&&{6}(99+1)+ \\&&{7}(9+1)+ \\&&{3} \end{eqnarray} $ Now if we distribute and rearrange, we get this: $ \begin{eqnarray} {272673}= &&\gray{2\cdot99999}+ \\&&\gray{7\cdot9999}+ \\&&\gray{2\cdot999}+ \\&&\gray{6\cdot99}+ \\&&\gray{7\cdot9}+ \\&& {2}+{7}+{2}+{6}+{7}+{3} \end{eqnarray} $ Any number consisting only of $9$ s is a multiple of $9$ , so the first five terms must all be multiples of $9$ That means that to figure out whether the original number is divisible by $9 $ , all we need to do is add up the digits and see if the sum is divisible by $9$ . In other words, ${272673}$ is divisible by $9$ if ${ 2}+{7}+{2}+{6}+{7}+{3}$ is divisible by $9$ Add the digits of ${272673}$ $ {2}+{7}+{2}+{6}+{7}+{3} = {27} $ If ${27}$ is divisible by $9$ , then ${272673}$ must also be divisible by $9$ ${27}$ is divisible by $9$, therefore ${272673}$ must also be divisible by $9$.